Digesting process and apparatus



May 19, 1936. T, l.. DUNBAR DIGESTING PROCESS AND APPARATUS 2 Sheets-Sheet l Filed Sept. `l, 1953 Wi n 22kt.

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May 19, 1936.

T. L. DUNBAR DIGESTING PROCESS AND APPARATUS Fiied sept. 1, 193s 2 Sheets-sheet 2 Patented May 19, 1936 PATENT OFFICE nrGEs'rlNc. PRocEss AND APPARATUS Thomas L. Dunbar, Watertown, N. Y., assigner to Chemipulp Process Inc., Watertown, N. Y., a corppration of New York Application September 1, 1933, Serial No. 687,861

32 Claims.

This invention relates to improvements in processesy and apparatus for digesting fibrous material in the making of paper pulp, and more particularly to improvements in the system disclosed in my U. S. Patent No. 1,669,234,` dated May 8, 1928. The present application is a continuation in part of my application Serial No. 664,630, filed April 5, 1933.

One of the objects of the invention is to provide a cooking system in which chemical liquor during the cooking loperation may be pumped directly from the upper portion to the bottom portion of a digester, and may be employed to withdraw gas from the top portion of the digester and to introduce the same in admixture with the liquor into the bottom portion of the digester.

Another object is to furnish a digesting apparatus including means -for withdrawing chemical liquor from a digester during the cooking operation, and for pumping the same through an eductor back into the bottom portion of the digester; such eductor functioning to withdraw gas from the top of the digester and to mix the same with the circulating liquor before the latter is returned to the bottom of the digester.

A further object is to supply an improved process and apparatus for continuously circulating gas and liquor from the top portion of a digester back tothe lower portion of the digester during the cooking operation.

Another object is to provide a digesting syst'em in which hot liquor at the end of the cookv ing process is forced through a closed or pressure vessel functioning as a heat exchanger for water which is heated in said exchanger, before it is introduced into the top of the digester.

` A still further object is to provide means for agitating the cooking liquor in said pressure vessel for releasing gases from the same.

` v-Another object is to furnish -an alternative process and apparatus whereby acid liquor may b'e circulated in an endless path through a digester and accumulator tank, so as to build up a condition in the digester to create a disassociation of water and water solubles or other incrustating and binding material from the raw material, which partially dilutes the acid before this acid is carried on into the accumulator where it may be preconditoned before it is returned to' the digester. This operation acts as a sort of scavenger to cleanse the raw fibrous material prior to the final starting of the cooking process whereby-when the later stages 'of the cooking process start, the raw material lies in a pool of fresh, clean liquor at a relatively higher temon until the air contained in the cells of the raw material has been substantially removed and replaced by the cooking liquor.

With the foregoing objects outlined and with other objects in view which will appear as the description proceeds, the invention consists in the novel features hereinafter described in detail, illustrated in the accompanying drawings, and more particularly pointed out in the appended claims.

In the drawings,

Fig. 1 is a diagrammatic view of my improved f apparatus.

Fig. 2 is a side elevation of one form of circulating means for a digester.

Fig. 3 is a similar view of another form of circulating means.

Most circulating systems so far proposed cirlate the cooking liquor only, but inV my above mentioned patent, I disclose means for circulating the gas from the top to the bottom of the digester, as well as means for also circulating the liquor. I have improved the system disclosed in my said patent in such a way as to make it unnecessary to employ an outside tank.

The circulating arrangement shown in Fig. 1 of the present application is the same as that shown in greater detail in Fig. 3, with the exception that the latter figure does not show the indirect heater for heating the liquor that is being circulated. Such indirect heater is purposely left out of Fig. 3, as circulation may take place both with and without the indirect heater. As a matter of fact, the arrangement 'shown in Fig. 3 is more effective for cooking purposes than other circulating systems where an indirect heater is used, but Where noy provision is made for taking the gases from the top of the digester, mixing them with the circulating liquor and then forcing the mixture into the bottom of the digester.

Referring particularly to Fig. 1, I designates an ordinary sulphite digester, and while only one digester is shown for the purpose of disclosing the invention, it will be understood that one or more digesters may be used in a plant, but each digester will be equipped the same as the one illustrated, and will be so arranged that the gases and liquors will -be vented and controlled from each digester. In case there is more than one digester in a plant, the line 22 will be a header that collects all the high pressure gases; line 28 will be a low pressure gas header, and 30 will be a header to take care of the gases vented during the time that the digester is being lled with liquor.

2 designates an acid accumulator tank, and 24 an acid storage tank. 51 is a pulp storage tank, 59 a head box, 60 a filter and 6I a pulp storage tank. I 31 is a degassing and water. heating tank, 42 is a hot water storage tank, and 50 is a conventional gas cooler su'ch as is used in conjunction with bi--sulphite acid making systems.

'Ihe above designated elements 'are suitably connected by valved pipes having interposed eductors, heaters, pumps, etc., which will be referred to in the following description vof the operation of the system.

According to one method of operation, after the digester has been filled with chips or the like from a conveyor or chip bin not shown, the man head or top cover plate is bolted into position and-proper connections made to the line 32 at the top o'f the digester. As soon as the man digester is continued until the temperature at the top of the digester is equalized with the temperature at the bottom of the same, and is continued normally until a temperature of from about to 100 C. is reached throughout the digester. Then the valves 8 and 9b are closed and the pump 4 is permitted to run until ahydrostatic pressure up to pounds has been placed on thedigester; this pressure being measured by the ordinary pressure gauge, not shown, which is normally located about on the same level as the n valve 8. As soon as this pressure has been reached, valve 7 is closed and steam is admitted into the bottom of the digester in the usual way, and as a result, the contents of the digester are gradually raised in temperature, and then the valves 8 and `9b are so manipulated that the proper cooking level oi liquid in the digester is reached between the temperatures of C. and 125 C., and this will be indicated by the gauge glass 63. At this time'the valves 8 and 9b are closed-andliquor and gas from the top of the digester is circulated to the bottom of the digester by the means shown in either Figs. 1, 2 or 3. In the construction head has been bolted on, the air vent valve 84 is qu.lilvn in Fig. l, the pump I6, when the valve opened while the valves 3Ia, 2|, I9, 52, 8, I4, 83 and 54 are kept closed. At such tne the valve 'l at the bottom of the digester, and the valve 3 at the bottom of the accumulator are opened. As the pressure in the accumulator 2 is normally from 30 to 40 pounds per square inch at the top, such pressure forces hot cooking liquor through valve 3, by-pass 5, pipe 6 and valve 1 into the bottom of the digester. As the pressure in the accumulator is largely a gas pressure, the forcing of the acid liquor into the digester gradually reduces the pressure in the accumulator, and this affects an automatic pressure regulating device 62 in the line 6, which automatically starts the motor, not shown, of'the pump 4, just as soon as the pressure in the line 6 reaches a predetermined point. The exact point at which this pressure regulating device starts into operation has to be determined for each individual plant, and is entirely dependent upon local conditions.

The pump is continued in operation until the digester is completely filled and a pressure of. up to 90 pounds has been `pumped on the digester. During lling, as soon as gas appears through the air vent valve 64, at the top of the digester, this valve is closed to prevent further escape of -gas into the atmosphere, and at the same time the valves 3Ia and 29 leading to the storage tank 24. are opened, while valves 21 and 3| are closed. The valves 3Ia and 29 remain open to pass acid gas to the storage tank until liquor starts to flow through the valve 29. This is easilyldetected by the different sound made by the owing liquor. That is, gas flowing through the valve 29 causes an entirely dierent sound than liquor, and consequently, the operator will know when to close the valve. At the time of closing the valve 29, and as soon as the pressure mentioned heretofore has been pumped on the digester, the valve 8 at the side of the digester is opened, and liquor will pass through the lines 9 and 9a, valve 9b and check valve I0, back into the accumulator, through pump II, heater I2 and eductor I3, while valves 9c and 9d are kept closed to prevent passage through lines 9e and 9f. At the same time the pump 4 is kept in operation, and liquoris forced from the accumulator into the bottom of thedigester. This circulation of liquor through the accumulator and I4 is opened, causes circulation of the liquor4 through pipe I5, heater I1 and eductor I8, and at the same time gas is withdrawn through valve I9 and pipe 20 and enters the eductor I8, in 53 1,669,234, such circulation of the gas, as well as the liquor, maintains the strength of the acidy in the digester during practically all of the cooking period. This means a great saving in time, as the stronger acid at the progressively higher temperature obtained during the cooking, produces a more active action on the lignin and otherv non-cellulose materials, and at the same time maintaining this stronger gas (as the gas is already heated), saves considerable consumption oi steam, and therefore reduces the dilution of the cooking liquor in the digester.

As soon as the proper temperature has been reached, depending upon the grade of pulp desired, the valve 2| is opened and the SO2 gases are vented through header 22 to the eductor 49 where they are mixed with a flowing stream of cool liquor forced by the pump 25 through line 26 from the storage tank through the drop leg 23 and eductor I3 into the accumulator. In the eductor I3, the mixture is admixed with liquor which may be recirculated from the bottom of the accumulator through valve 64 and check valve 65 by the pump II. As there are normally several digesters in a plant, it will be readily understood that the pump 25 is kept in continuous operation, and there is always a flowing stream of cool liquor through the eductor 49 to take care of gases vented from the digesters through header 22.

in U. S. Patent 1,888,632, dated Nov. 22, 1932. 'I'he valve 21 is leftiopen until the pressure on the digester has been reduced to any point decided upon or to atmospheric, and then this valve is closed, and it may be stated that in normal operation, as soon as valve 21 is opened, valves I9 and I4 are closed.

When practically all of the gas has been removed from the digester, the valve 33 at the bottom of the same is opened, and then the pump 34 is set in operation to force the liquor from the digester through line 35 into a degasslng tank 31 which is of closed construction so as to maintain a pressure therein. The tank 31 is provided with an auxiliary chamber 36 containing a shaft and propeller, indicated by the numeral 53. The wa'll of the chamber is perforated so that agitation vcaused by rotation of the propeller causes degassing of the liquor, and this gas is vented off through line 44, gas cooler 50 and line 45 to 'a siphon 45a, which receives a flowing stream of acid from the acid system, not shown, by means of a pipe 45h. The mixture flows from the siphon through a drop leg 45e into the acid storage tank 24. The cooler 50 may be cooled by a heat exchange coil, not shown, arranged within the same and receiving cold 'water from a pipe 46. Water heated in such a coil will be discharged through a pipe 41 and may be sent to the towers used for acid preparation. Degassed liquor will be discharged from the. tank 31 through a line 38.

By means of pipe 44a, and valves 44b and 44e, the gases from tank 31 may be by-passed around the cooler 50 and be fed in heated condition to the siphon 45a.

While the liquor is being pumped' out of the digester by the pump 34, water is admitted into the top of the digester through the valve 52. 'I'his water enters the system through pipe 39 having valve I, and it passes through a heat exchange coil 40 in the tank 31, so that it is heated in this tank by outgoing hot liquor from the digesters. From the coil, the hot water passes through the pipe 4| into the hot water storage tank 42, from which it may be passed through the pipe 43 and valve 52 into the top of the digester. As the gauge glass 63 extends the whole length of the digester, the displacement of the spent cooking liquor-in the digester by the hot water can be observed in the gauge glass. As soon as the latter indicates the water has displaced the cooking liquor, the pump 55 is started, and the contents of the digester are pumped through valve 54 and line 56 into the pulp storage tank 51; 'I'his tank is provided with a valve 58 so that ow of materials from the tank to the flow box or mixing tank 59 can be regulated. From said box, the pulp flows to the continuous pulp filter 60, where it is th'oroughly washed and then deposited in the tank 6 I. This arrangement may bev similar to the one shown 4in the patent to yCharles R. Van de Carr, Jr., No. 1,818,913, dated August 11, 1931.

Hot water from the storage tank 42 may be run through a valved pipe 43a to a hot well or other hot water storage, so it may be used as-boiler feed water, if desired.

The operation of the degassing tank 31 is similar to that disclosed in my application Serial No.

.605,033, with the exception that'in the present case, the degassing takes place under pressure, due to the fact that the tank is closed.

I'he lines 9, and I5 .are provided at their inlet ends with a strainer 66 which is arranged within the digester, and it will beunderstood that all lines leading from the digester, with the exception of the line 56, may be provided with strainers, not shown, to permit the withdrawal of fluid and to prevent the passage of flbrous material.

A pipe 48 may be employed to vent gas from the storage tank 24 into the line 45, and another pip'e 65 maybe used to vent gas from the top of the accumulator into the storage tank 24. A gauge glass l2 may be used on the accumulator.

In the alternative method of operation, the hot acid from the accumulator is forced by the pump 4 into any one of the digesters recently filled with chips. the Valve 64 until acid gas appears at said valve, and then the valve is closed and the valve 29 opened and the gas is conducted by the line 30 back to the storage tank. As soon as liquor appears at the top, all valves, both side relief and top -relief, are closed, and a pressure of from to 90 pounds per square inch is pumped on the digester by the pump 4. As soon as the predetermined prcssure has been reached, both the side relief and top relief valves 8 and 2l of the digester areopened andthe pump is continued in operation, pumping liquor out of the accumulator into the digester back through the lines9, 9e and 22 into the eductor 49 through the drop leg 23 and eductor I3 back to the accumulator. This pumping operation is continued until the temperature has `been substantially equalized at both the top and bottom of the digester and until the air in the cells of the chips or the like has been replaced by fresh, clean cooking liquor. Meanwhile, steam is admitted if necessary, either into the line 22 through valve 2I a or into the accumulator through valved-pipe 2a, or into the digester through the regular steam lines. As soon as the temperatures have been equalized at the top and bottom of the digester and the air removed, which in the average plant will require somewhere be'- tween minutes to 41/2 hours, the valves 8 and 2| are again closed and the contents of the digester brought up to about l0 to 90 pounds again by the operation of the pump 4. Steam is then admitted to the digester and cookng continued in the usual way with the venting of relief fluids 'from the digester.

During the circulation above mentioned, instead of passing the liquor from the digester back to the accumulator by using the eductor 49, the liquor can be passed directly into the accumulator by causing the same to flow through the lines 9h and 9f, and of course, by properly manipulating the valves in the lines, such liquor may flow through 8, 9, 9h and 9f to the accumulator. Normally the side reliefs and top reliefs are rather small and the desired results could, therefore, be accomplished by the extra line Bf, 9h which takes care of the circulation. Although this is somewhat more' expensive and in many places it might be necessary to use equpment already installed. I desire it to be understood that this auxiliary line 9j, 9h can of course be used in my system.

The real function of the pumping of pressure on the digester and subsequent circulation is to build up the temperature of the acid high enough to create a disassociation of water and water solubles' or other incrustating and binding substances from the raw material which partially dilutes the acid and then this acid is carried on into the accumulator for preconditioning before it is returned to the digester, although the cycle of circulation is constant until the desired results have been accomplished. In other words, this operation acts in a measure as a scavenger to cleanse the raw material of certain substances This digester is vented to the air through and the air from the cells thereof prior to the final startin of the ycooking process so that when the later st ges of the cooking process start, the raw material lies in a pool of fresh clean liquor at a relatively high temperature.

The results obtained by operating this way are quite remarkable in that actual yields of 52 to 54% of pulp are being obtainedl as compared with the old cold acid operations which show a variation in yield of from 42 to 45%. At one of the plants where I have installed such a system, it 'actually shows a further saving of two dollars and fifty cent ($2.50) per ton over and above the saving originally accomplished with a known conventional hot acid system. This is probably due to the cleansing effect on the raw material and the added advantage of equally distributing the heat unit throughout the entire mass of material to be cooked. Referring-to Fig. 2, the digester la has a valved liquid outlet pipe lia through which the liquor can be withdrawn by the'pump l6a. This liquor is circulated by the pump through the pipe I6b which leads from the discharge side of the pump to the bottom portion of the digester. A gasoutlet pipe 20a leads from the top portion of the digester to an eductor I8a. which is interposed in the pipe I6b. It will be understood that during the cooking operation, when the valves in the pipes I5a, Ib and 20a. are open and the pump l6a is in operation, liquor will be pumped from the upper portion of the digester into the lower portion of the digester, and in so circulating it will draw gases from the top of the digester, mix the same with' the circulating liquor, and return the admixture to thebottom of the digester.

In the circulating system shown in Fig. 3, the digester Ib has a liquid outlet pipe I5b leading from -its upper portion to a p ump I6c. The discharge side of this pump is connected to a pipe iSd that leads to the bottom of the digester, and has an eductor Illb'interposed therein. A gas line 20h leads from the top of the digester to the eductor, and it will be understood that when the valves in the pipes are opened and the pump I6c is in operation, the liquor and gases will be circulated in the same manner as described in connection with Figs. 1 and 2.

From the foregoing it is believed that the invention may be readily understood by those skilled in the art, and it is manifest that changes may be made in the details disclosed, without departing from the spirit of the invention, as expressed in the claims.

What is claimed and desired to be secured by Letters Patent is:

1. In the digestion of fibrous material, pumping acid liquor during the cooking operation continuously and directly from the upper portion to the bottom portion of the digester, and utilizing such pumping operation for-simultaneously withdrawing gases directly from the upper portion of the digester for admxing the same with the withdrawn liquor and for returning said gases in ad. mixture with the liquor back into the bottom portion of the digester.

2. In the digestion of fibrous material, pumping acid liquor duringthe cooking operation continuously and directly from the upper portion to the lower portion o f the digester,-causing said liquor in its travel to pass through an eductor,

a leading gases from the top portion of the digester to said eductor, and mixing ,said gases in the eductor with the circulating liquor and returning the mixture to the bottom portion of the digester.

3. An apparatus of the character described. comprising a digester, a pump arranged exteriorly of the digester, a. pipe connecting the 5 upper portion of the digester with the inlet of the pump, a second pipe connecting the outlet of the pump to the bottom portion of the digester, an eductor interposed in the second pipe, and a third pipe connecting the top portion of the digester to said eductor. i

4. An apparatus of the character described, comprising a digester, a pump arranged exteriorly of the digester, a pipe connecting the upper portion of the digester with the inlet of the pump, 15 a second pipe connecting the outlet of the pump to the bottom portion of the digester, an eductor interposed in the second pipe, and a third pipe connecting the top portion of the digester to said eductor, all of said pipes, and the eductor 20 being positioned exteriorly of the digester.

5. In the digestion of fibrous material, charging a digester with fibrous material, introducing hot acid liquor under its own.pressure into the digester, subsequently pumping more hot liquor 25 into the digester, and afterwards continuously circulating gas and liquor'directly from the upper portion to the lower portion of the digester, and mixing said gas and liquor while passing the same from the upper to the lower portion of the 30 digester.

6. In a process of the character described, charging a digester with fibrous material, storing hot acid liquor in an accumulator, and then circulating hot acid liquor from the accumulator 3 through the digester and back to the accumulator until said iibrous material is heated to a predetermined degree.

'7. In a process of the character described, charging a digester with lbrousmaterial, storing hot acid liquor in an accumulator, and then circulating hot acid liquor from the accumulator through the digester and back to the accumulator until said fibrous material is heated to a predetermined degree, of from about80 to 100 C. 45

8. In a process of the character described, charging a digester with fibrous material, storing hot acid liquor in an accumulator, then circulating hot acid liquor from the accumulator through the digester and back to the accumulator until said fibrous material is heated to a predetermined degree, and heating the circulating liquor while it is returning from the digester to the accumulator.

9. In a process of the character described, cir- 55 culating acid liquor from an accumulator back into an accumulator, passing the circulating liquor exteriorly of the accumulator through a heater and eductor,passing cold acid liquor through a second eductor and then into the first mentioned eductor, and passing vent gas from a cooking operation into the second eductor and admixing the same with the cold acid liquor in the second eductor.

10. In a process of the character described, charging a digester with fibrous material, charging an accumulator with hot acid liquor, circulating said hot acid liquor from the accumulator through the fibrous material in the digester and back to said accumulator until the contents of the digester are heated to a predetermined degree, and subsequently continuously circulating gases and liquor from the upper portion to' the lower portion of the digester.

11. In a process of the character described, 75

' ing an accumulator with hot acid liquor, circulating said hot acid liquor from the accumulator through the fibrous material in the digester and back to said accumulator until the contents of the digester are heated to a predetermined degree of about 80 to 100 C., and subsequently continuously circulating. gases and liquor from the upper portion to the lower portion of the digester.

12. In a process of the character described. pumping acid liquor from the upper portion to the bottom portion of a digester during a cooking operation, heating the flowing liquor and .passing the same through an eductor on its way back to the digester, and utilizing the action of said eductor for withdrawing gases from the top of the digester and 'for mixing `the same with the liquor flowing back to the digester.

13. In a process of the character described,` charging a digester with fibrous material, charging an accumulator with hot acid liquor, then using the pressure of the liquor in the accumulator to cause the hot acid liquor to travel through a passageway into the digester, and utilizing the pressure existing in the passageway for controlling the operation of a pump, said pump being used for subsequently forcing hot acid liquor from the accumulator into the digester.

14. In a process of the character described, passing hot spent acid liquor from a digester at the end of the cooking period through a pressure tank, and passing water in heat exchange relationship with the hot liquor through said tank and into the top of the digester for displacing the spent liquor from the digester.

l5. In a process of the character described, passing hot spent acid liquor from a digester at the end of the cooking period through a pressure tank, passing water in heat exchange relationship with the hot liquor through said tank and into the top of the digester for displacing the spent liquor from the digester, agitating the liquor in said pressure tank for separating gas from the same, and discharging said gas from the tank separate from said spent liquor.

16. An apparatus of the class described, comprising a digester, an accumulator, a valved pipe connecting the accumulator to the digester and having a pump interposed therein, a by-pass around said pump, and means controlled by pressure conditions in said pipe for controlling said pump.

17..An apparatus of the class described, comprising a digester, an accumulator, a valved pipe connecting the digester and accumulator, a pump interposed in said pipe for forcing hot acid liquor from the accumulator to the digester, a second valved pipe for leading acid liquor 'from the digester to the accumulator, and a pump interposed in the second pipe.

18. An apparatus of the class described, comprising a digester, an accumulator, a valved pipe connecting the digester and accumulator, a pump interposed in said pipe for forcing hot acid liquor from the accumulator to the digester, a second valved pipe for leading acid liquor from the digester to the accumulator, a pump interposed in the second pipe, and a heater interposed in said tank, means for leading a gas stream out of the upper portion of said tank, and a. pipe for leading spent acid liquor from the lower portion of said tank.

20. In a `process of the character described, 5 charging a digester with fibrous material, intro-l ducing treating liquor into the digester until the latter is completely filled with the fibrous material and liquor, then sealing the digester and pumping additional liquor into the digester until a predetermined superatmospheric pressure isV pumped on the digester, afterwards unsealing the digester, and then circulating such liquor through the digester.

21. In a process of the character described, charging a digester with fibrous material, introducing treating liquor into the digester until the latter is completely fllled with the brous material and liquor, then sealing the digester and pumping additional liquor into the digester until a predetermined superatmospheric pressure is pumped on the digester, afterwards unsealing the digester, and then circulating such liquor through the digester until the temperature throughout the n fibrous material in the digester is substantially equalized and raised to a substantially predetermined degree. Y

22. In a process of the character described, charging a digester with brous material, passing hot acid liquor into the digester until the latter is completely filled with the fibrous material and acid liquor, then sealing the digester and pumping hot acid .liquor into the digester until a predetermined superatmospheric pressure is pumped on the digester, afterwards unsealing the digester, then circulating hot acid liquor through the digester until the temperature throughout the fibrous material in the digester is substantially equalized and raised to a substantially predetermined degree, subsequently again sealingthe digester, and then pumping hot acid liquor into the digester until a pressure of up to 90 pounds per square inch gauge is created within the digester.

v23. In a process of the character described, charging a digester with fibrous material, forcing hot chemical liquor into the digester and in contact with the fibrous material while permitting air and gases to escape from the digester, then after the digester is completely filled with hot chemical liquor and fibrous material, sealing the digester and continuing the forcing of hot chemical liquor into the digester until a predetermined superatmospheric pressure exists within the digester, subsequently unsealing the digester and circulating hot chemical liquor through the digester until a predetermined temperature exists in the fibrous material substantially through the digester, and then resealing the digester and forcing hot chemical liquor into the same until a predetermined superatmospheric pressure is obtained within the digester.

24. In a process of the character described, charging a digester with brous material, forcing hot chemical liquor into the digester and in contact with the fibrous material while permitting air and gases to escape from the digester, then after the digester is completely lled with hot chemical liquor and fibrous material, sealing the digester and continuing the forcing of hot l in the fibrous material substantially' throughout the digester. then reseaiing the digester and torcing hot chemical liquor into the same until a predetermined superatmospheric pressure is obtained within the digester, and heating the chemical liquor used in the digester between the ilrst sealing and second sealing steps.

25. In a process of the character described, charging a digester with nbrous material, forcing liquor into the digester while permitting the escape of air and gases, until the digester is completely filled with brous material and liquor, sealing the digester and continuing the forcing of liquor into the same until a predetermined superatmospheric pressure is obtained within the digester, afterwards unsealing the digester and forcing liquor through the digester and into contact with the fibrous material and thereby releasing air from the brous material and creating a disassociation of water and water solubles or other incrustating and binding substances from the fibrous material, diluting the liquor in the digester with said substances and then passing the diluted liquor into a tank, preconditioning the diluted liquor in the tank, and then returning said preconditioned liquor to the digester.

26. In a. process of the character described, charging a digester with brous material', forcing hot chemical liquor into the digester while permitting the escape of air and gases, until the digester is completely lied with iibrous material and chemical liquor, sealing the digester and containuing the forcing of hot chemical liquor into the same until a predetermined superatmospheric pressure is obtained within the digester, afterwards unsealing the digester and forcing hot chemical liquor through the digester and into contact with the brous material and thereby releasing air from the fibrous material and creating a disassociation of water and water solubles or other incrustating and binding substances from the brous material, diluting the chemical liquor in the digester with said substances and then passing the diluted liquor into a tank, preconditioning the diluted chemical liquor in the tank, and then returning said preconditiofned liquor to the digester.

27. In a process vof the character described, charging a digester with brous material, introducing treating liquor into the digester until the latter is completely iilledwith the brous material and liquor, then sealing the digester and pumping additional liquor into the same until a pressure is predetermined superatmospheric pumped on the digester, afterwards unsealing the digester, and then circulating such liquor from a tank through the digester and back into said tank.

28. In a process of the character described, passing hot spent liquor from a digester at the end of the cooking period into a pressure tank, agitating the liquor in said tank for separating gas from the same, passing said gas from the tank to a Siphon, and passing acid from an acid system into said Siphon, co-mingling said acid and gas in the Siphon, and feeding the mixture from the siphon into an acid liquor storage chamber.

29. In the digestion of brousmaterial, pumping acid liquor during the cooking operation continuously and directly from the upper portion to the bottom portion of the digester while maintaining the liquor in a restricted stream, and utilizing such pumping operation for simultaneously withdrawing gases directly from the upper portion of the digester for admixing the same with the withdrawn liquor and for returning said gases in admixture with the liquor back into the bottom portion of the digester.

30. The method of digesting cellulosic material which comprises subjecting the material to the action of a gas evolving liquor and maintaining the liquor at normal strength except as it may be weakened by its reaction with the cellulosic material vby heating `the liquor indirectly and by causing'the gas evolved from the liquor to be reabsorbed thereby. A, v

31. A digesting system for cellulosic material comprising a digester, a heat exchanger, means for circulating a gas evolving digester liquor between said digester and heat exchanger, and means for returning the evolved gas into the liquor.

32. A digester system for cellulosic material comprising a digester adapted to receive the material and a charge of gas evolving liquor, a heat exchanger, means to circulate the liquor between the digester and the heat exchanger to heat the liquor,- means for liberating the digester of the evolved gas, and means for introducing gas into the circulatory path of the liquor exteriorly of the digester.

THOMAS L. DUNBAR. 

